I think the fallacy here is to assume that anyone working on
the project needs to be an as top-notch programmer as you are.
Usually, that's not the case.
Many projects only need "expert" knowledge for a fraction
of the project, but the rest is just "grunt" work.

Good point.
For fun, let's analyze a specific scenario.
Let's assume that the "Perl 6 project" is "late" and
that we have some (below average) developers available to
throw at it with the goal of "finishing" it sooner.
For this hypothetical scenario, let's loosely define "finished"
as all Synopses
complete and all implemented in at least one Perl 6
implementation with similar performance and stability to perl 5.14.
I don't want to get distracted with nit-picking the "definition
of done" here. The point of this little exercise is to
gain insight into Brooks' Law: where does it hold,
where should it be repealed?
To simulate what I typically see in commercial projects,
let's further assume that these new developers have
little prior knowledge or experience in Perl.

Now, adding new "below average" developers to perform
highly skilled work, such as finishing
the Synopses
or improving the Perl 6 parser seems counter-productive
to me, a classic case where Brooks' Law holds.
In which of the following areas would adding more
people help finish the project sooner?

I may have missed some key areas above or broken things down wrongly.
Please feel free to suggest improvements.

I suspect that most open source projects (such as Perl 6) are
well ahead of most commercial ones in terms of documentation
and in partitioning the system into independent components.
Accordingly, I feel Brooks' Law is less applicable to open
source projects than to commercial ones.